Centronuclear Myopathies (CNM) are a group of congenital myopathies characterized by muscle weakness and confirmed histologically by fiber atrophy, predominance of type I fibers, and increased centralization of nuclei, not secondary to muscle regeneration. Three main forms of CNM have been characterized: X-linked CNM (XLCNM also called myotubular myopathy, OMIM 310400) due to mutations in the phosphoinositides phosphatase myotubularin (MTM1) (Laporte, J. et al., Nature Genetics, 1996. 13(2): p. 175-82), autosomal recessive CNM (ARCNM, OMIM 255200) caused by mutations in the membrane remodeling protein amphiphysin 2 (BIN1) (Nicot, A. S. et al., Nature Genetics, 2007. 39(9): p. 1134-9), and autosomal dominant CNM (ADCNM, OMIM 160150) due to mutations in dynamin 2 (DNM2) (Bitoun, M. et al., Nature Genetics, 2005. 37(11): p. 1207-9) or due to mutations in other genes, such as BIN1 (Bohm et al., Brain. 2014 Sep. 25. pii: awu272. [Epub ahead of print]). Other genes have been linked to a CNM-like myopathy: RYR1 encoding for the ryanodine receptor, TTN encoding for Titin, CCDC78 (OMIM 614807) and the phosphoinositides phosphatase MTMR14 (called hJUMPY; OMIM 160150). The genetic relationship between the implicated genes is not known and potent therapeutic approaches are lacking.
X-linked centronuclear myopathy, also called myotubular myopathy, is the most common and severe form of CNM, with neonatal onset and death often occurring in the first years of life (Jungbluth, H. et al., Orphanet J Rare Dis, 2008. 3: p. 26). There is currently no cure, nor effective treatments available for this disorder. To date more than 200 different mutations in MTM1 have been reported in about 450 families, most of which lead to a strong reduction of protein. Mtm1 knockout or knockin mice have previously been characterized, which recapitulate the CNM phenotype with classical histological features including abnormal organelle positioning, mislocalization of nuclei and muscle atrophy, associated with a corresponding reduction in muscle strength. A defect in triads structure associated with abnormal excitation-contraction coupling has been detected in several animal models and patients with different forms of CNM, identifying a common defect in all CNM forms (Defects in amphiphysin 2 (BIN1) and triads in several forms of centronuclear myopathies, Toussaint A. et al., Acta Neuropathol. 2011 February; 121(2):253-66). This is consistent with a proposed role of MTM1 in the regulation of phosphoinositides level on the sarcoplasmic reticulum component of the triads.
Dynamins are large GTPase proteins that play important roles in membrane trafficking and endocytosis, and in actin cytoskeleton assembly. Dynamin proteins contain an N-terminal GTPase domain, middle domain, PH domain (phosphoinositide binding), GED (GTPase effector domain), and a PRD (Proline-rich domain) for protein-protein interactions. Three human dynamins have been identified; dynamin 1, expressed exclusively in neurons, dynamin 3 predominantly in brain and testis, and dynamin 2 (DNM2) which is ubiquitously expressed. Different heterozygous DNM2 mutations have been identified in tissue-specific diseases: Autosomal Dominant Centronuclear Myopathy which affects skeletal muscle, and autosomal dominant Charcot-Marie-Tooth (CMTDIB, OMIM 606482) peripheral neuropathy.
Recent biochemical studies have suggested that some CNM-causing DNM2 mutations increase the dynamin oligomer stability and GTPase activity. This was complemented in vivo by either knockin or over-expression of the most common CNM-DNM2 patient mutation (p.R465W) in mice, which induced CNM-like features in adult mice, indicating the disease is not due to haploinsufficiency. Over-expression of wild type (WT) DNM2 also caused perturbation to the muscle, albeit to a lesser extent.
The patent application WO 2013/0065558 describes that miR-133a plays a modulatory role on DNM2 expression. As DNM2 is mutated in CNM, it is stated that an agonist of a miR-133 family member might be beneficial for the treatment of centronuclear myopathy. However, miR-133 has numerous targets (see online databases for miRNA target prediction and functional annotations, such as http://mirdb.org/miRDB/ where 226 predicted targets for hsa-miR-133a in miRDB are given and none of them is DNM2, the same types of results are obtained with other online databases). Since miR-133 has numerous targets and is therefore not selective, it could have deleterious effects. Moreover, improvement of CNM with miR133 delivery has not been reported so far.
Accordingly, there is a significant need for an appropriate centronuclear myopathy treatment, in particular for new and more effective therapeutic agents.